The Need for Shielding

Author: N.H. Crowhurst

One of the problems of audio amplifiers is the fact that audio voltages and currents are often so small that the circuits carrying them have to be protected against unwanted voltages and currents induced from other circuits. There are three kinds of unwanted induction: power, crosstalk, and feedback.

The need for shielding

All kinds of power components and circuits radiate at power frequencies (mostly 60 and 120 cycles). If this radiation is induced into a low-level audio circuit, hum will result. The second kind of induction is that of high-level circuit, if the two are in close proximity to each other. (This interference is known as crosstalk.) If the same audio at high level is induced in low-level circuit, it is the same as undesired feedback, which can cause instability or oscillation.

Transformers, chockes and current-carrying circuits radiate fluctuating magnetic fields which induce a voltage in any conductor placed in the field.

The solution for all of these troubles is shielding to protect the low-level circuit against pickup due to magnetic or electric fields. Any transformer will radiate some magnetic field. Audio transformers radiate a magnetic field that contains audio, and power transformers radiate at power frequencies. (Chokes can also radiate at these frequencies.) These magnetic fields can induce voltage in low-level circuits by magnetic induction. This works by causing a voltage to be induced (in the same way that a transfer of energy occurs in a transformer) whenever the field changes or fluctuates. An electric field is caused by the presence of high voltages. Any high-level circuit carrying high audio or power voltages will radiate an electric field that can induce charges in a low-level audio circuit, producing pickup.

Electric induction